1. Field of the Invention
The invention relates to a method and apparatus for transmitting digital signals and in particular to signal multiplexing techniques employing pulse stuffing.
2. Description of the Prior Art
A digital signal multiplex device of the type described in an aritcle of the German magazine "Frequenz", 32 (1978) 10, pages 281-287, entitled Positive-Zero-Negative Stuffing Technique for Multiplex Transmission of Plesioschronous Data Signals, combines a plurality of individual digital signals of a given hierarchy stage to form a digital signal of the next higher hierarchy stage. The bit rates and the pulse frames of the digital signals of the individual hierarchy stages are laid down by international agreements. At all levels, the transmission takes place plesiochronously (nearly-synchronously). As used herein, the term "plesiochronous digital signal" means a digital signal having a bit rate which can exceed or fall below its nominal bit rate by a maximum of one tolerance value. The relative tolerances are likewise laid down. An n-channel multiplex device for n-plesiochronous data signals consists of a multiplexer which combines the n-digital signals to form a multiplex signal, and of a demultiplexer which separates the multiplex signal into n-digital signals. The method whereby the bit rates of the plesiochronous digital signals are matched to the bit rate of the multiplex signal is known as pulse stuffing (or justification). A known method in this context is the positive-zero-negative stuffing method corresponding to the United Nations established International Telephone and Telephony Consultative Committee (C.C.I.T.T.) recommendation G.702.
A multiplex device for plesiochronous digital signals consists of clock rate matching units which are individually assigned to each of the plesiochronous channels and of a synchronous multiplexer. The synchronous multiplexer makes available to each of the plesiochronous channels two synchronous channels for the transmission of the digital signals, a main channel, and an auxiliary channel having a very much lower bit rate. The bit rate of the main and auxiliary channels are in a fixed rational ratio to the bit rate of the multiplex channel.
In the case of positive stuffing, the value of the bit rate of the digital signals in the plesiochronous channels is lower than the value of the bit rate of the main channel. The clock rate matching unit of a channel transfers the digital signal in question, bit by bit, to the associated main channel until a phase difference corresponding to the period of one bit has been reached as a result of the difference between the bit rates. Then, at a suitable point in time, the clock rate matching unit inserts a blank bit, the so-called stuffing bit, into the digital signal. At the same time the clock rate matching unit transmits an item of stuffing information via the auxiliary channel. The item of stuffing information indicates the point at which the stuffing bit is inserted.
In the case of negative stuffing, the value of the bit rate of the digital signals in the plesiochronous channels is greater than the value of the bit rate of the main channel. In this case, at a suitable point in time in the multiplexer, an information bit is extracted from the digital signal and is transmitted via the auxiliary channel together with the stuffing information. In the demultiplexer this information bit is re-inserted into the digital signal at the appropriate position in accordance with the stuffing information.
The fundamental construction of multiplex devices, in particular the transmitter, is described in an article entitled "Multiplexer for 8.448 Mbit/s in a Positive-Negative Stuffing Technology" by U Assmus et al. published in the German magazine "Nachrichtentechnischen Fachberichten", 42 (1972), page 245-256 incorporated herein by reference for illustrating the state of the art. In a central cocmponent, the transmitter contains a clock generator for the generation of the multiplex frequency and devices for frame generation and nesting. Individually in respect of each channel, each input system is assigned a phase comparator, a format converter, and a circuit for the selection of the necessary clock rate matching information. The frame generating device emits frame words on the basis of which the sum signal emitted from the multiplexer acquires a frame structure.
In these known methods the stuffing command or stuffing information is transmitted via the auxiliary channel. An unrecognized stuffing command leads to the repetition or loss of a bit at the receiving end. Therefore, an unrecognized stuffing command results in a longer shift of the data by one bit and in a corresponding series of faults. When a stuffing command remains unrecognized because of adulterated transmission, the loss of synchronism of the data can be recognized only on the basis of the shift of the frame word.
U.S. Pat. No. 4,489,421 issued to Erich Burger on Dec. 18, 1984 discloses a digital communications transmission system in which, at the transmitting end, two plesiochronous data signals, including additional or auxiliary signals thereof, are combined to form a multiplex signal or sum signal. One of the additional signals contains the instantaneous phase relationship between the two plesiochronous bit repetition frequencies of the data signals as binary phase words. The phase relationship, as transmitted by the binary phase words, is used to regenerate the plesiochronous clock signal at the receiving end of the transmission link. Although it is in fact known to transmit phase words; these phase words are not used as stuffing information. The repetition frequency of the phase words in additional channel is not selected to be sufficiently high that stuffing commands can be derived therefrom. It is noted that in postal networks, re-synchronization takes place relatively rapidly but not in networks in which encoded transmission is used, in which case re-synchronization takes place much slower.